The invention relates to a display tube comprising an envelope portion having an inner surface provided with a first electrically conductive layer and a faceplate having an inner surface provided with a second electrically conductive layer, which faceplate is secured to the envelope portion, and further comprising connection means to electrically connect said first electrically conductive layer to second electrically conductive layer.
A display tube of the type mentioned in the opening paragraph is disclosed in U.S. Pat. No. 5,087,855 (PHN 13.021), which discloses a cathode ray tube having an envelope portion with a high-voltage contact extending through the wall of said envelope portion, and a faceplate which is sealed to the envelope portion. The envelope is internally coated with a first electrically conductive layer and the faceplate is coated with a second electrically conductive layer. The high-voltage contact consists of an anode button being projected inwardly. A connection means having two tongue-shaped members is fastened to the anode button at the inner side of the envelope portion. The tongue-shaped members extend on both sides of the high-voltage contact and engage the respective first and second electrically conductive layers.
A display tube of the type defined in the opening paragraph is generally used for displaying monochrome pictures, color pictures or for displaying figures or letters (Data Graphic Displays). In practice, it was, however, found that in some cases the display of a picture is not always produced in the desired manner so that detrimental effects may occur. Particularly, when the display tube is subjected to vibrations and/or high accelerations, the display of a picture may be disturbed.
It is inter alia an object of the invention to provide a display tube in which the occurrence of said detrimental effects is prevented at least in a number of cases.
According to the invention, a display tube of the type defined in the opening paragraph is therefore characterized in that said connection means comprise a contacting layer which is provided in an area partially overlapping the first electrically conductive layer and the second electrically conductive layer.
The invention is based on the recognition that the detrimental effects occur at least partially because the first electrically conductive layer and the second electrically conductive layer are not electrically connected to a sufficient extent, and the supply of a voltage to the layers is not adequate in some cases. Particularly the use of mechanical means, such as the tongue-shaped members in U.S. Pat. No. 5,087,855, in order to electrically connect the first and the second electrically conductive layer may yield contact disruptions when the display tube is subjected to vibrations and/or high accelerations.
An insufficient electric connection between the first and the second conductive layer may yield unwanted electromagnetic fields. On their path to the display screen electron beams generated by an electron-generating system provided in the envelope portion can detrimentally be influenced by these electromagnetic fields. This causes an uncontrollable and unwanted shift of the electron beams which results in a distortion of a picture to be displayed.
The display tube of the invention has been found to ensure an adequate electric connection between the first electrically conductive layer and the second electrically conductive layer because of the very stable connection provided by the contacting layer. This electric contact ensures a good operation of the display tube even without additional mechanical connection means. Even when the only connection means is the contacting layer, which is deposited in an area partially overlapping the first electrically conductive layer and the second electrically conductive layer, than good operation of the display tube can thus be ensured.
When the first or the second electrically conductive layer has a high resistivity, the overlap of the contacting layer with said layer having a high resistivity is preferably such that the area of overlap is at least 1 cm2. Because of the high resistivity, which in most cases is several kilo Ohms, a too small overlap area would result in local heating of the layers which is unfavourable for the lifetime of the contacting layer and thus for the lifetime of the display tube.
When the first or the second electrically conductive layer has a high conductivity, the overlap of the contacting layer with said layer having a high conductivity is less critical. As the contacting layer usually also has a high conductivity, a small overlap area is sufficient to establish an electric contact according to the invention.
Preferably, the contacting layer has a thickness of more than 10 nm. A very thin layer may result in an unreliable electric contact or in heating of said layer, which heating is unfavourable for the lifetime of the contacting layer and thus for the lifetime of the display tube. On the other hand, a too thick contacting layer may scale off, yielding loose particles in the display tube, which should be avoided.
The contacting layer according to the invention adheres to the underlying material. It has been found that the use of mechanical means that press against the first and the second electrically conductive layer, as commonly used, may introduce loose particles in the display tube due to scratching of the internal layers by the mechanical contact. This problem is also reduced in the display tube of the invention.
The electrical resistance of the electric interconnection of the first and the second electrically conductive layer by the contacting layer is preferably less than 100 xcexa9. Typically, when using a material with a high conductivity such as Ba having a layer thickness of approximately 100 nm, the electrical resistance of the electric interconnection of the first and the second electrically conductive layers by the contacting layer is approximately 1xcexa9.
A way to establish the electric connection according to the invention is provided by the following method of electrically connecting a first electrically conductive layer is provided on the inner side of an envelope portion of a display tube and a second electrically conductive layer provided on the inner side of a faceplate of the display tube, which faceplate is secured to the envelope portion at a sealing edge, by depositing a contacting layer of a conductive material at the inner side of the display tube, which conductive material is deposited in an area bridging the sealing edge and electrically contacting the first electrically conductive layer and the second electrically conductive layer.
Preferably said contacting layer does not only have a good adhesion to the first and the second electrically conductive layer, but also to the material in between, which material may comprise glass and frit material. Said frit material is used to seal the envelope portion to the faceplate. The frit material is a semi-crystalline glass comprising PbO, ZnO and SiO2. The semi-crystalline glass comprises at least one crystalline phase in a matrix of a glass phase.
Although the contacting layer may be deposited by simply painting an electrically conductive paint or printing an electrically conductive layer, it is preferred that the contacting layer is provided by vacuum deposition. Vacuum deposition may also comprise sputtering. Using vacuum deposition, an electrically conductive layer can be deposited after the display tube has been evacuated. When the vacuum pumping of the display tube has been completed, the electrically conductive material is evaporated to produce a cloud that moves towards the sealing edge, and a thin layer of electrically conductive material is deposited at least in an area bridging the sealing edge and electrically contacting the first electrically conductive layer and the second electrically conductive layer.
Preferably, the contacting layer is not deposited on the screen section of the faceplate. A deposition on the screen should be avoided because this may have an unfavourable influence on the brightness of the display.
The electrically conductive material forming the contacting layer preferably comprises an electrically conductive getter material and/or a metal. The getter material may comprise a material chosen from the group comprising Ba, Ti, Zr, or mixtures of these materials. In order to bind residual gases in the evacuated display tube, a getter material (in the form of a pill) such as Ba or Ti, may be evaporated to produce a fine spread of getter material, called the xe2x80x98getter mirrorxe2x80x99. In colour display tubes, metallic Ba is commonly used as a getter. A Ba getter mirror is electrically conductive and may be used for bridging the sealing edge and electrically contacting the first electrically conductive layer and the second electrically conductive layer.
Although the use of a Ba getter mirror for binding residual gases is well known, this getter mirror is not deposited at the sealing edge, because the deposition at the sealing edge is blocked by an Internal Magnetic Shield (IMS), as shown in DE 19542263 for example. The IMS is made of a ferromagnetic material and is intended to substantially shield the electron beam from the earth""s magnetic field.
An embodiment of the method according to the invention requires an area adjacent to a position on the sealing edge, including at least a small area of the first and the second electrically conductive layer, to be covered with getter material which is evaporated when the getter pill is heated.
A preferred embodiment of the display tube according to the invention is a display tube comprising an internal magnetic shield having at least one opening, wherein the opening in the internal magnetic shield is located so as to allow deposition of the contacting layer on the sealing edge. Preferably, said display tube further comprises a getter pill, wherein the opening in the internal magnetic shield is located with respect to the position of the getter pill so as to allow deposition of the contacting layer comprising getter material, which getter material originates from the getter pill. This may be achieved, for example, by making cut-outs in the IMS. When said area adjacent to a position on the""sealing edge is visible from the position of the getter pill, the evaporated Ba covers this area with a Ba getter mirror. This metallic mirror then provides a conductive electric contact between the first and the second electrically conductive layer.
Another preferred embodiment of the display tube according to the invention is a display tube comprising a deflector positioned on the getter pill receptacle, wherein the deflector is positioned so as to allow deposition of the contacting layer comprising getter material, which getter material originates from a getter pill. As an alternative to the opening in the IMS, a deflector may be positioned on the getter pill receptacle, which deflector directs a part of the evaporated getter material towards the sealing edge. Due to the deflection of the evaporated getter material, a contacting layer is deposited, which contacting layer provides an electric contact between the first electrically conductive layer and the second electrically conductive layer.